Your dog should always enter the car from the pavement side
Keep your dog on a lead until he is inside the car and has supervision
Discourage your dog from jumping out of the car as soon as you are home - he may do this in a busy area
Make it obvious to other drivers that your dog is under control when entering/ exiting your car.
More Tips
Using a crate that your dog is used to will help your dog feel secure. If you have to use the passenger seat, you can buy harnesses to fit your dog. Always make sure crates and carriers are placed somewhere they will not move around. If you are traveling for more than two hours, your dog should have been exercised before the journey. You should also make frequent stops to allow it to relieve itself and for a drink of fresh water.
Never leave your dog in the car when it is hot. The temperature inside the car can reach unbearable temperatures and it is by no means uncommon for dogs to die of heat exhaustion when left like this. Leaving windows open makes no difference - to be effective this would mean opening them so wide that your dog could escape anyway. Likewise, sun shades are simply not effective enough to prevent intolerable heat build-up.